One knee aliases

BUR #756 has been rejected.

create alias on_one_knee -> one_knee

Two tags for the same thing.

BUR #759 has been approved by @Penance.

create alias one_knee -> on_one_knee

Danbooru alternative. Related: https://danbooru.donmai.us/forum_topics/23459

The bulk update request #759 (forum #4179) has been approved by @Penance.

The bulk update request #756 (forum #4174) has been rejected by @Penance.